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Old July 11th, 2004, 05:25 PM   #1
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Condenser lens kills DOF with agus35?

hi- I'm building an agus35, and have encountered some problems while doing so. These problems might be insignificant to a weathered professional....but that's not me. Anyway, I finished my agus 35 and discovered a distracting light spot right smack in the middle of my image. So I ran a search and asked around. The solution seemed to be placing a "condenser lens" between GG and the dv camera. I ripped one out of an old slide projector and put it in. After doing so I read and discovered myself that this lens compromised 35mm DOF. Now, I've heard of using medium format lenses and two condenser lenses to correct this problem...but obviously price is an obstacle. Is there a practical solution?
thanks for any help -alex
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Old July 12th, 2004, 12:37 AM   #2
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A quick and dirty solution for the hotspot is to go in closer and use a smaller area of the groundglass, ie., academy frame instead of the still image 35mm frame.

The downside is any resolution and surface defects on the groundglass are amplified by the higher magnification of the smaller area.

The wider the available aperture of the SLR lens on front, the better for the hotspot problem. Use neutral density gel between the lens and the groundglass for excessive diffusion and flare across the disk.

This is a bit tricky as it might bring back the hot spot problem a little but leaves you with better depth of field effects. Downide is a softer image too if the SLR lens is not up to scratch on wide aperture setting.

Some SLR lenses are better than others. Also as the texture of the groundglass is finer and there begins a blend of aerial image and projected image into the camcorder, the hotspot will become aggravated and a faint sharp detail inside soft detail outside of the focal plane selected with become evident.
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Old July 12th, 2004, 02:08 AM   #3
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Condenser WILL NOT effect DOF on GG

Alex-
You either heard wrong or missunderstood. A condenser between the GG and the video cameras lens WILL NOT effect the DOF.

Take it from me because I've done it all with this project. In fact I was the one that recomended using condensers soooo long ago. A condenser can do many things like magnify a image or allow you to focus even closer on a object (ie diopter) or it can be used to reduce the hot spot on the GG. It important to have a basic understanding in optics if you plan on making one of these adapters but basically you WANT the video cameras lens to keep the image is sees on the GG to in perfect focus. The shallow DOF is happening on the other side of the GG by the 35mm lens. Now it is possible that by putting a condenser in between the GG and the camcorder to reduce the hot spot you could also change the minimum focus to be even shorter as well. If you put your GG too close to the camcorder you are going to run into DOF problems because you will not be able to keep the entire GG in focus with your camcorders lens at one time.

I COMPLETE confused everyone huh....Just read the first two lines again and nevermind the rest. ;-)

-Brett Erskine
www.CinematographerReels.com
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Old July 12th, 2004, 11:44 AM   #4
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Ah yes, sorry for my ignorance...but I guess I need to learn somewhere. In fact, I think I'll go look for books about optics in the book forum right now.
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Old July 12th, 2004, 09:05 PM   #5
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I apologise for my own ignorance. My use of the words "go in closer" is easyspeak for zoom in closer which may necessitate use of a higher power close-up lens or diopter.

As Brett correctly points out, positioning the camera too close to the groundglass screen does introduce another problem, a ring of soft focus around the outside of the image or a patch of soft focus in the center if you adjust the outside to sharpness.

My setup with the SW5042 lens set is extremely close, like about 18mm. This is great for close coupling but introduces the hotspot and peripheral soft focus faults. This arrangement is temporary until I get a prism erector sorted, then the maginification power of the close-up lens will come back to 4+.

Brett.

By the way, the SW5042 lens set is three elements, one a bonded composite of two pieces and a third single piece, a very thick powerful convex element.
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